FIG. 1 shows the arrangement of a conventional optical pickup device which includes a light source (e.g. a semiconductor laser) 1, a parallel planar plate 2, an objective lens 3, a recording medium (e.g. an optical disk) 4, and a photo-detector 5. As shown by the arrows of FIG. 1, a divergent light beam is emitted from the light source 1 to the surface of the parallel planar plate 2 where the light beam is reflected to the objective lens 3. The objective lens 3 serves to focus the reflected light beam onto the optical disk 4.
The focused light beam on disk 4 is then reflected to the photo-detector 5 through the objective lens 3 and the parallel planar plate 2. Since the parallel planar plate 2 consists of a glass plate or the like, and since plate 2 is disposed in the light beam converging path between the objective lens 3 and the photo-detector 5, the light beam received by the photo-detector 5 necessarily suffers from astigmatism.
FIGS. 2(a)-(e) show the photo-detector 5 of FIG. 1, wherein the photo-detector is divided into four regions D.sub.1 -D.sub.4 by a first straight line 51, and a second straight line 52 which is perpendicular to the first straight lie 51. When the light beam from the objective lens is correctly focused into the disk 4, a so-called "focus state" is achieved. More particularly, in the focus state, a substantially circular light spot is formed on the photo-detector 5 as shown in FIG. 2(a). However, when the disk 4 is moved towards or away from the lens 3, a substantially elliptical light spot is formed on the photo-detector as shown in FIGS. 2(b) and 2(c). Therefore, according to the principle of astigmatism, a focus error signal having a characteristic as indicated by the curve a in FIG. 3 can be obtained. The focus error (FE) signal associated with the light spots of the conventional pickup device can be detected from the output of the photo-detector 5. More particularly, the focus error signal is equal to the difference between the sum of the outputs of regions D.sub.1 and D.sub.3 and the sum of the outputs of regions D.sub.2 and D.sub.4. Curve a of FIG. 3 shows the focus error signal of the light spots of FIGS. 2(a), 2(b), and 2(c), wherein the minimum point of curve a is the focus error of FIG. 2(b), the origin is the focus error of FIG. 2(a) (i.e. no focus error), and the maximum point of signal a is the focus error of FIG. 2(c). In addition to detecting the focus error signal, the photo-detector 5 can detect variations in the level of the reflected light beam from beam 4 (which is modulated with signals (pits) recorded on the disk) from the sum of the outputs of regions D.sub.1 -D.sub.4 of photo-detector 5.
A problem occurs with the detection of the focus error signal of the pickup device of FIG. 1 when the optical components of the pickup device are not properly positioned, or when the objective lens 3 is moved in a tracking direction. More particularly, in either of these situations, the light spot reflected onto the photo-detector 5 from the disk 4 will be shifted from the center of the photo-detector 5 (FIG. 2(a)) to, for example, the position shown in FIG. 2(d). As a result, the focus error signal becomes offset (from the origin) as shown by curve b of FIG. 3. In addition, the situation may arise where the objective lens is moved such that the light spot has the shape and position as shown in FIG. 2(e). Although the light spot of FIG. 2(e) has a zero focus error signal, it is not in the focus state. As a result, the degree of modulation of the light beam by the signals (pits) recorded on the disk 4 is lowered, thereby causing the pickup device to be sensitive to noise.
Thus, there is a continuing need in the art for a pickup device which can provide a correct focus error signal even when the light beam applied to the photo-detector is somewhat shifted from a predetermined reference position on the photo-detector.